The formation of hydrazoic acid HN 3 is inherent to many azide processes due to the presence of small amounts of protic components in the reaction mixtures. Hydrazoic acid is an unstable component which may decompose violently. To ensure safe working conditions during the development or the production of azides, the lower decomposition limit (LDL) of this substance in a nitrogen atmosphere was determined using a 5-L explosion sphere. No decomposition could be observed for HN 3 concentrations below 10%. Moreover, the influence of solvent vapors was investigated to demonstrate that they can be used to inhibit the decomposition reaction of hydrazoic acid.
A quantitative on-line NIR (near-infrared) method for the monitoring of the formation of a Grignard reagent was developed on laboratory scale using chemometrics and tested in a 630 L pilot plant reactor. Good accuracy of the model was obtained, and feasibility of the on-line measurement was demonstrated. The concentrations of the added reagent (and thus its degree of accumulation) and of the formed Grignard reagent (indication of the reaction initiation) can be determined in real time. Therefore, the safety of the highly exothermic process and its robustness are significantly improved. Furthermore, the method is applicable to monitoring concentrations during the following cross-coupling reaction so that it can be used to determine in real time the reaction yield. By using on-line spectroscopy, the product quality and performance can be guaranteed on an industrial scale. Moreover, the cycle times are reduced due to the elimination of waiting times caused by traditional analysis.
Hydrazoic acid (HN 3 ) is formed during the synthesis of tributyltin azide and in the following cycloaddition to prepare a tetrazole ring compound. The formation of this substance is inherent to many azide processes because small amounts of protic components cannot be avoided. Hydrazoic acid is a very toxic volatile compound, which has highly explosive properties. Under certain conditions (presence of impurities in the reaction mixture), the gas concentration can come close to the decomposition limit. Therefore, online monitoring of this gas concentration is essential to ensure the process safety. FT-IR and FT-NIR experiments were performed in the laboratory scale to calibrate the spectrometers. Due to the possibility of using quartz light fiber cables, a FT-NIR spectrometer was installed to monitor the hydrazoic acid concentration in the industrial scale. * To whom correspondence should be addressed: Building WSJ-145.11.54, Scheme 1. Synthesis of tri-n-butyltin azide Scheme 2. Cycloaddition of tri-n-butyltin azide to a nitrile to form a tetrazole ring compound Scheme 3. Postulated mechanism of the formation of hydrazoic acid in presence of protic components 2 HN 3 f H 2 + 3 N 2
Drying is an important part of manufacturing processes in the chemical and pharmaceutical industry. The product quality often depends on the drying conditions and efficiency. Moreover, this unit operation frequently represents the bottleneck of the whole process. Therefore, online monitoring of the drying operation can lead to a significant improvement of the cycle time as well as to a reduction of the analytical work and costs. This paper demonstrates the effectiveness of NIR spectroscopy to control drying processes on an industrial scale. The examples shown in this study are realized using different kinds of dryers (filter dryer, paddle dryer, and spherical dryer) and various solvents (water and organic solvents). The spectral data are evaluated using multivariate calibration methods (partial least squares regression). These studies show that the solvent concentrations determined using NIR are in good agreement with the reference analyses. Moreover, the direct measurement in the powder allows stopping the drying process at a given residual solvent concentration (which is crucial in certain processes, e.g. to obtain specific hydrate forms). All together, the implementation of NIR for online monitoring of drying processes leads to a significant optimization of plant equipment utilization and to a higher throughput while reducing the risk of out-of-specifications batches.
The utilization of online spectroscopic methods (FTIR/Raman) for the development and the monitoring of chemical processes was demonstrated with two important chemical reaction types. The synthesis of tri-n-butyltin azide takes place at the solidliquid interface and is therefore very sensitive to changes of reaction conditions or surface properties. Traditional offline analysis should be avoided due to the high toxicity of the compounds. The concentration variations of all involved compounds were determined. In the same way, the synthesis of 2-chloroaniline was studied to detect the formation of an unstable intermediate (the hydroxylamine) which can lead to uncontrolled decomposition reactions. Online spectroscopy is an opportunity to obtain the information about a reaction system faster and more efficiently than with conventional methods. This allows the rapid optimization of safe chemical processes and provides information for process control.
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